The present invention concerns a plastic coated steel tube, comprising at least one plastic layer containing a hydrolyzable silane. The purpose of the silane-containing layer is to bond the plastic coating onto steel or epoxy-coated steel. Moreover, the silane-containing plastic layer is cross-linked which improves its mechanical and thermal properties.
Plastic coated steel tubes have previously been produced, e.g. for transporting natural gas, the steel tube being first shot-blasted and cleaned, and thereafter heated with a gas flame or in an induction furnace to 90.degree. to 230.degree. C., to be thereafter coated with a black polyethylene compound and with a hot melt or an ethylene polymer containing acid (the acid may be doped or copolymerized) for adhesion layer.
This two-layer coating process can be carried out with both layers being co-extruded or tandem-extruded, the tube being passed through a die (cross-head coating), or so that the tube rotates at the same time while a film of molten plastic is extruded thereupon (spiral coating). It is also common to apply the adhesion layer on the tube by powder coating. It is important, in any case, that the adhesion between plastic and steel is sufficiently good to exclude entry of moisture which would corrode the steel.
Recently, epoxy has also been introduced for corrosion protection of steel tubes, because epoxy is highly adhesive to steel. On the other hand, epoxy is rather sensitive to impact and therefore an epoxy-coated steel tube is usually additionally coated with a black polyethylene compound and with an acid-containing ethylene polymer as an adhesion layer. This three-layer coating (MAPEC technique) is accomplished principally in the same manner as a two-layer coating, with the exception that epoxy is coated as powder or liquid immediately before the adhesion layer coating. The coating with these layers has to be very closely controlled. The epoxy must not be too insufficiently cured (too low a temperature or too short a time), or too greatly cured (too high a temperature or too long a time) when the adhesion layer is applied, in order to obtain the desired adhesion (the requirements are considerably higher than for two-layer coating). Additionally, the temperature of the adhesion plastic must be sufficiently high (over 210.degree. C.).
The three-layer coating has been introduced on the basis of increasing demand imposed in various countries. In addition to the increased requirements concerning the level of adhesion, requirements are also imposed on aging strength, adhesion at low or high temperatures, etc. These demands are standardized in various countries. The adhesion plastics for two-layer coating and three-layer coating of steel tubes currently available on the market, are based on polyethylene and polypropylene or on their copolymers grafted with low content (less than 1%) of unsaturated acid (e.g. maleic acid anhydride) or ethylene copolymers or terpolymers containing high amounts (over 4%) of unsaturated acid (e.g., acrylic acid or maleic acid anhydride).
The traditional adhesion plastics are aggressive in the extrusion process, and require well-controlled conditions when coating on steel or epoxy. Moreover, these adhesion plastics usually melt at fairly low temperature (not thermally stable), and they are not cross-linkable.
Silanes are traditionally used on monomers when producing silicones, and a so-called coupling agents in order to improve the adhesion between, e.g., fillers, glass fiber, and plastic.
In the plastics industry, silane-grafted polyolefins or ethylene-silane polymers or terpolymers are also used for cross-linking under the influence of a silanol condensation catalyst and water. Silane-grafted polyolefins and ethylene-silane copolymers or terpolymers also exhibit good adhesion to metals, such as steel and aluminum, as well as to polar plastics, such as polyamide, polyester, etc., while the adhesion can be improved by blending in organic acids or amino silanes.